It is well known to introduce water marks on paper. The application of watermarks has hitherto been done in a variety of ways. One such method is to form the watermarks during the manufacture of the paper by means of round moulds bearing impressions which are recessed and/or in relief, with the aid of watermarking rollers bearing designs which are recessed and/or, raised, associated with a flat table (Fourdrinier machine). This method then makes it possible to obtain an image which, when the sheet of paper is viewed in transmitted light, appears either as a light impression (when using a roller with a relief design) or as a dark impression (when using a roller with a recessed design). The clear areas are due to the fact that the thickness of the sheet and the density of the fibers are less than the thickness of the sheet and the corresponding areas where the roller has not left an impression.
A further method is to create pseudo-watermarks by printing or depositing a compound, generally greasy, which renders the sheet of paper permanently transparent. The transparent effect can equally well be created by means of a heat-fusible material such as polyethylene as described in EP-A-203499. In the latter reference, the pseudo-watermark is applied to a sheet of paper which comprises a thermally sensitive material, and heat is applied to a part of the surface of the paper in a manner to cause a region of the paper to become irreversibly translucent.
Yet another method of creating a watermark on a sheet of paper is to control the opacity of pre-defined areas of the paper using an opacifier. FR-A-2353676 describes such a process. The opacifier may be an aqueous suspension of a pigment or other chemical compound which in turn may be a coloured compound or an ink. The opacifier is applied during the manufacture of the sheet, on the fibrous nap and before it is raised from the backcloth, in such a manner that said opacifier penetrates into the interstices of the nap at predefined areas, thereby changing the opacity thereof upon drying. This technique has the disadvantage that it requires special roller arrangements to apply the opacifier and equipment to enable the opacifier to penetrate the nap interstices.
Again U.S. Pat. No. 4,520,063 describes a sheet made of synthetic resin containing security markings which are similar to watermarks. Two cross-linkable colourants, which have different rates of migration into the surface of a sheet in the presence of a cross-linking agent, are applied on the sheet. Subsequent cross-linking of the colourant when the desired migration depth has been reached, by the colourant penetrating at least a part of the thickness of the sheet to correspond to the selected security feature eg pattern or motif, shows the security feature in congruant fashion on both sides of the sheet. In reflected light, a motif can be seen which bears the colour of this colourant on the surface of the sheet, and in transmitted light a motif of mixed colour can be seen. This process is difficult to apply on a plastic sheet since it requires careful choice of unobvious colourants and cross-linking agents.
A further process is described in EP-A-500726 which describes a process for producing a printable flexible sheet. This sheet incorporates at least one substrate sheet of synthetic material having at least one surface treated with a compound to modify its opacity at pre-determined points in such a way that when viewed by transmitted light, a distinctive security mark can be seen. This compound is then covered by a printable pigmented layer.
As can be seen, most of the above processes are rather complex and involve the use of a numerous additional compounds, colourants, cross-linking agents and/or a plurality of layers. Synthetic papers based on fibrous materials are intrinsically capable of being given a watermark in the same way as cellulosic papers, so that the mark becomes an essential feature of the sheet which is difficult to reproduce.
For synthetic paper based on film, hitherto the methods used have been applied to the synthetic paper film directly or to a fill which is converted to a synthetic paper by coating, as described above. The watermark, as applied in such cases, has essentially its final dimensions and form.
It has now been found that where manufacture of synthetic paper includes a stretching and/or orientation stage(s), suitable modifications of the ingoing sheet prior to orientation by the use of heated rollers or by pressure can produce variations of opacity in the finished sheet which gives the effect of a "watermark" in conventional paper.